Finely divided silica, such as silica hydrogel or amorphous, precipitated silica, is used as a flatting agent to lower the gloss of coating formulations such as lacquers. However, the silica flatting agent tends to settle out of certain formulations upon standing. When a can of certain nitrocellulose lacquers, for example, is stored, a heavy, semi-solid or hard material collects in the bottom of the can. Before the lacquer can be used, the settled material must be dispersed through the lacquer. At times, the settled material cannot be adequately dispersed even with vigorous stirring.
U.S. Pat. No. 2,625,492 discloses that silica hydrogel flatting agents treated with an inorganic, fluorine containing compound, notably magnesium silicofluoride (also called magnesium fluosilicate), settle in softer, more easily dispersible layers than untreated flatting agent. In the described process, a hydrogel is formed by rapid acidification of an aqueous solution of sodium silicate. The gel is then broken up, treated with aqueous ammonia, and an aqueous solution of magnesium fluosilicate is circulated through the gel fragments until they attain a desired fluorine content. The treated gel is then dried, activated by heating, and ground to the desired particle size for use as flatting agent. The patent states broadly that the silica may be treated with any fluorine-containing compound at other stages of the operation, but only treatment in aqueous medium is described.